The present invention relates to a device for mixing and delivering orthopaedic bone cement or the like.
Orthopaedic bone cement is used throughout the world to secure hip, knee and other anatomic prostheses in an appropriate anatomical position. The bone cement is produced by thoroughly mixing together two components, usually methylmethacrylate monomer liquid and polymethylmethacrylate powder. The mixing was previously carried out using a simple bowl and spatula. The surgeon then removes the required amount of cement and manipulates it by hand before inserting it into a preformed cavity or applying it to a resected bony surface where the prosthesis is to be positioned. Cement may either be applied by hand or may be put into a syringe and applied thereby.
Several improvements have been made to this mixing arrangement, including providing arrangements for mixing under vacuum, and to improve the mixing efficiency, to result in a homogenous cement material.
Several devices for mixing cement, usually in a vacuum, are presently available and in general use.
Of the available devices, the preferred forms are the ‘bowl’ type mixers and the ‘syringe’ mixers.
Bowl type mixers are provided usually in the form of hand-held mixing bowls. The substances to be mixed are placed in the bowl to which a vacuum is applied. The substances are mixed by means of a rotating paddle extending into the bowl, which is rotated manually by means of a handle extending through the lid of the bowl. In some applications, the use of such a mixing bowl, an example of which is disclosed in EP-A-0616552, is favoured. Many surgeons prefer to ‘hand pack’ the cement. Bowl mixing also tends to be preferred by nurses who are used to the convenience of mixing in this vessel. A bowl is easier to use and it is important that the nurses feel confident, since timing is very crucial and the mixture must be ‘right first time’. Many surgeons also tend to prefer bowl mixers. It is crucial that the mixture does not begin to set before it is applied and experienced surgeons can tell, by touch, when the cement is at the right stage for applying to the bone cavity.
These ‘bowl’ mixers are in widespread use and are very popular. They are easy to use, allowing repeatable consistent mixing, independent of the level of skill of the user, and the concept of mixing used by the bowl is simple and is popular with nurses. The bowl type mixer is very flexible in that it can be used to mix all types of cement and can be used to mix varying quantities of cement. In bowl mixers where a high vacuum is applied, the cement has low porosity and thus high strength.
In the bowl mixer of EP-A-0616552, which has a ‘rotating axis’, as opposed to the paddle having a fixed axis, the cement is very thoroughly mixed and the chances of ‘dead spots’ or areas of unmixed cement occurring are very small.
In some applications, it is preferable or even necessary, to apply the mixed cement to the bone or bone cavity by means of a syringe. Indeed, many surgeons prefer syringe-type application to ‘hand packing’.
If, for such applications, the cement is initially mixed in a bowl as described above, it must then be transferred to a dispensing syringe. This transfer can be messy and time-consuming and may expose the mixture to more air entrapment. The introduction of air into the cement produces a weak cement, which has obvious disadvantages.
To overcome this problem, mixing devices have been designed which combine a mixing chamber and a syringe. For example, EP-A-0178658 discloses a device for mixing bone cement comprising a mixing container connected to a feed device. A vacuum source is connected to the feed device for mixing the substances under vacuum. This device has proved to be a very efficient mixing and transfer system and eliminates the need to transfer the mixed cement from the mixing bowl to a syringe.
U.S. Pat. No. 4,758,096 and U.S. Pat. No. 3,606,094 also disclose bone cement mixers in which the cement is mixed in the dispensing vessel itself. In the first of these patents, the mixing is effected manually by means of a ‘masher’ plate-type agitator. The masher plate is attached to a shaft attached to a handle. The agitator is moveable in the chamber both axially and rotatably to permit mixing of the cement by the user, moving the handle vertically and rotatably. However, such mixing operation is difficult and inefficient and does not guarantee thorough mixing of the cement. Partial strokes of the ‘masher’ can lead to areas of unmixed powder and the mixing is not consistent, and is reliant on the consistency of the user.
Another problem with the ‘masher’ type system is that it is difficult to mix standard viscosity cement using this plunger. As the plate is pushed down into the cement, it meets a high resistance, which can result in only a partial mixing stroke being carried out and the cement being compacted at the base of the mixing chamber.
Other, improved mix-in-the-syringe mixers are disclosed in, for example, DE-C-883326 and EP-A-0744991.
Again, these mix-in-the-syringe mixers have become very popular and are in widespread use.
The mix-in-the-syringe mixers are very useful where relatively small quantities of cement are mixed and used. Most mixers on the market are designed to be able to mix up to a ‘double’ mix of the highest volume cement currently in common use (e. g. Simplex Cement).
Several different types of cement are commonly used in orthopaedic applications. These cements have very different characteristics and volumes, and also have different viscosities. Table 1 below shows three of the most commonly used cement types, showing approximate volumes of dry powder, and the corresponding volume of mixed cement.
Cement Volumes:
Dry Powder VolumeMixed CementCement Typemm2Volume mm2SIMPLEX (USA)302,000135,000PALACOS125,000130,000CMW130,000139,000
From the above table, it can be seen that cements such as SIMPLEX have a dry powder volume roughly three times that of other cements, to produce the same volume of mixed cement.
Thus, generally, for mix-in-the-syringe mixers, the mixing chamber must be large enough to receive, say, a double mix of dry powder of the highest volume cement commonly used, even though the actual volume of mixed cement is considerably less. This means that generally the body of the syringe or the mixing chamber is, in fact, much larger than necessary for other types of cement, and requires a longer mixing stroke than would be required for the other low volume cements.
In some cases, the surgeon will wish to prepare an even larger quantity of cement, e. g. a triple mix. Larger amounts of cement are required, for example, in a revision operation, or for certain types of primary hips.
Such larger quantities can generally be mixed in a bowl-type mixer, as described, for example, in EP-A-0616552. However, with the syringe-type mixers, it is generally not possible to merely increase the dimensions of the mixing chamber, to allow more cement to be mixed.
Increasing the height of the mixing chamber to accommodate the larger quantity of cement is not feasible, as this results in a device which is just too big to handle comfortably. The longer the chamber is, the more difficult it becomes to introduce the mixing paddle through the column in a correct alignment and to locate the paddle correctly. This is particularly so when the cement becomes more dense and it becomes extremely difficult to push the paddle down and to cause effective mixing.
Thus, there is a need for an effective and efficient, easy to handle, mix-in-the-syringe type orthopaedic cement mixer which is not unnecessarily big or which is capable of mixing large, e. g. triple, mixes of cement, even when the cement used is a large volume cement, such an simplex.
One mixer which aims to deal with this problem is produced by Stryker, and is described in U.S. Pat. No. 5,558,136 and associated patents. This mixer comprises a funnel section, leading into a cylindrical syringe body mixing chamber. A mixing paddle extends through the funnel section and is rotated by means of a handle in the lid of the funnel section. A large quantity of powder can then be inserted into the device through the funnel. When the monomer is added, and the cement powder and monomer mix, the resulting mixture has a smaller volume which is accommodated within the syringe body. The funnel part may then be removed, leaving a simple syringe body for attachment to a syringe gun and nozzle. However, this device relies on the use of low viscosity cement which, as mixed, falls, under gravity, into the syringe body mixing chamber. It is also essential that the mixing phase is started straight away, as soon as the monomer is added, before the mixture starts to ‘dough up’.
A problem is that unmixed powder can be left in the funnel as the cement reduces, which does not fall into the syringe body mixing chamber.
This can result in the unmixed powder falling into the mixed cement as the funnel part is removed, resulting in dry or dead spots and, thus, a brittle cement.
Another problem is that when mixing standard cements or where mixing is not started straight away, the doughing-up cement can stick to the walls of the funnel and the paddle causing high wastage.